During the last two decades environmental and economical demands have contributed to the increased use of recycled fibers in the production of paper. In the converting of paper raw material into pulp, recovered paper is broken down to fibers in a pulping process. In the pulping process, the paper is torn up and treated with chemicals at enhanced temperature. The fiber suspension leaving the pulper is further treated in several steps, including e.g. screening, flotation and/or centrifugal cleaning, and it may be bleached by the addition of oxidative and/or reductive bleaching agents.
One problem with the use of recycled fibers is the variety of contaminants in recovered paper. Many of these contaminants adhere to fibers and fillers therefore may cause problems during the recycling process. One of such contaminants are “stickies”. Stickies are grouped into primary and secondary stickies depending on their source and formation process, respectively. Primary stickies are introduced with the recovered paper while secondary stickies are produced by physicochemical processes during the process for producing recycled fibre pulp. Stickies may originate from a variety of different sources such as adhesives applications from various paper products, hotmelts from bookbinders and carton sealants, self adhesive labels, polymeric binders from coated paper and ink residues. Stickies typically are classified as hot melts, pressure-sensitive adhesives (PSAs), latexes, binders, pitch, and ink and combinations thereof. Stickies can also be classified as macrostickies or microstickies depending upon the size of the stickies.
Stickies may cause operational and product quality problems. Stickies can deposit on surfaces of the paper machine equipment, such as wires, felts, press rolls, and drying cylinders, cause process upsets, hinder fiber bonding, and reduce product quality. For instance, stickies may cause bright and dark spots in the paper or even sheet breaks, which results in lower paper quality, production losses and extra operating stops for cleaning. Consequently, it is necessary to monitor and control stickies to improve papermaking operations and product quality.
The amount of stickies in the recycled pulp may be reduced by, e.g., optimization of re-pulping parameters, cleaning and screening, flotation, dispersing and chemical treatment. Usual chemical treatments to reduce stickies or to reduce their sticky character are dispersion and/or passivation of the stickies by addition of e.g. talc and/or bentonite, chemical detacifiers, e.g. polymers, and treatment of the process equipments with chemicals to retard the deposition. These chemical treatments contribute significantly to operating costs.
In the art, the amount of micro- and macrostickies is usually determined using standard laboratory analytical procedures, e.g. extraction methods or laboratory screening. Per definition, microstickies are stickies that pass a 100 μm slotted plate in laboratory screening. Macrostickies are defined as the stickies in the residue of the 100 μm screen. The standard laboratory analytical procedures for determining sticky levels allow the determination of either micro-, macro- or both types of stickies.
For instance, macrostickies can be monitored using a device known as a “Pulmac shive analyzer” or “Haindl fractionator”, which screens out the macrostickies from the furnish and determines the macrosticky level through image analysis. This process usually takes several hours to complete, and is useful as a retroactive control of the macrosticky level.
The use of near infrared (NIR) spectroscopy in combination with multivariate data analysis to determine the amount of stickies has also been suggested. For instance, Enrico Pigorsch and Petra Behnsen (PTS Heidenau, journal ATIP, vol. 64, no 4) have suggested online monitoring of microsticky load in the paper web by NIR measurements.
Most standard laboratory analytical procedures for determining stickies are, however, laborious and time consuming and do not allow a continuous monitoring of the sticky load in the pulp to be processed. Moreover, they are limited to the determination of stickies of a certain kind (i.e. stickies of a certain chemical nature or composition), e.g. hot melts, pressure-sensitive adhesives (PSAs), latexes, binders, pitch, or ink; or of a certain size (i.e. micro- or macrostickies). As a result, a single standard laboratory analytical procedure for determining stickies cannot explain all effects and problems that may be caused by stickies, since each procedure is based on a different measuring principle taking size and chemical properties of the stickies into account.
There does not appear to be any method or system for continuously and automatically determining sticky levels of “all” kinds of stickies in a process for producing recycled fibre pulp.
Consequently, there is a need for a comprehensive method for continuously and automatically determining sticky levels in a process for producing recycled fibre pulp. In particular, there is a need for a method that allows determining various kinds of stickies with a single measurement easily and efficiently.